Recently, the present inventor described the preparation of bioelastomers which exhibit elastomeric force development which can be varied either as a function of temperature or as a function of the hydrophobicity of the repeating units of the elastomeric polymer. By reversibly changing the hydrophobicity of the polymer repeating units using a reversible chemical process such as protonation/deprotonation, the temperature of the inverse temperature transition of the bioelastomers can be reversibly shifted, thereby, in effect, turning "on" and "off" elastomeric force.
For example, in pending application Ser. No. 07/062,557, incorporated herein in its entirety, it was disclosed that by changing the relative hydrophobicity of the elastin polypentapeptide (PPP) repeating unit --VPGVG--.sub.n, using an occasional functional group, such as glutamic acid, the temperature of the inverse temperature transition can be lowered by protonation of a carboxylate side chain, for example, and can be raised by deprotonating the carboxylic acid group to form the anion. Of course, when modifying the hydrophobicity, it is necessary to do so in a way such that elasticity is retained. This may be accomplished, for example, by substitution at position 4 in the PPP repeating unit. With such a polymer, lowering the pH causes contraction and raising the pH causes relaxation.
Alternatively, the development of elastomeric force in different elastomers can be varied as a function of temperature by changing the hydrophobicity using Ile in place of Val, for example, at position 1. For example, in pending application Ser. No. 900,895, it was disclosed that cross-linked elastin polypentapeptide (PPP) and polytetrapeptide (PTP) and analogs thereof exhibit elastomeric force development at different temperatures spanning a range of up to about 75.degree. C. depending upon several controllable variables. Ser. No. 900,895 is also incorporated herein in the entirety.
Using these elastomeric systems, the present inventor has described in Ser. No. 062,557 the construction of molecular machines utilizing entropic motive force in elastomeric protein systems. As a simple example, a weight may be suspended from a synthetic elastomeric PPP band at 20.degree. C. in water. The band is formed on .gamma.-irradiation of --Val.sup.1 -Pro.sup.2 -Gly.sup.3 -Val.sup.4 -Gly.sup.5 --.sub.n, where n&gt;100 and the composition is approximately 40% peptide and 60% water by weight at 40.degree. C. On raising the temperature from 20.degree. C. to 40.degree. C., the weight is raised against gravity to a fraction of the 20.degree. C. length depending upon the load. Thus, for a band 10 cm in length, an appropriate weight would be raised 3 cm, for example, against the pull of gravity.
The immediately preceding example is one of converting thermal energy into mechanical work, i.e., thermomechanical transduction. However, it has now been found possible to design mechanochemical engines whereby mechanical work may be reversibly converted into chemical work, i.e., mechanochemical transduction. In accordance with the present invention, mechanochemical engines are provided using modulable inverse temperature transitions in the reversible interconversion of chemical and mechanical work.
For some years, it has been known that high salt concentrations can be used to lower the transition temperature for thermal denaturation of proteins. These salts are believed to interfere with electrostatic interactions in proteins, and also to increase the solubility of the peptide bond moiety in water. See Proteins: Structures and Molecular Properties (Freeman 1984). Using this principle, it has also been shown that a belt of collagen can be made to contract on introduction into a medium of high salt concentration. In 1965, for example, a mechanochemical engine was designed based upon the reversible contraction of partially cross-linked collagen by treating the collagen with a strong aqueous solution of lithium bromide, potassium thiocyanate or urea. By washing the fibers with water or dilute solution, the process was reversed, leading to elongation or relaxation of the collagen. However, as relatively high heats of denaturation are involved, large concentration gradients are required. In a description of such a mechanochemical engine in Katchalsky et al, Nature, pages 568-571 (May 7, 1966), two baths were used having a concentration of 11.25 M LiBr with a second bath having a concentration of no more than 0.3 M LiBr.
In such a mechanochemical engine the contraction-relaxation cycle can be repeated many times by bringing the collagen fiber sequentially into contact with one very dilute and then with a second very concentrated solution of LiBr. During this process, the solutions become mixed and the net mechanical work obtained in such a cyclical operation relates to the free energy of mixing of these solutions. In such an engine, it is the free-energy change due to the transfer of salt from the concentration solution to the dilute solution, and to the associated water flow, that is converted partly into mechanical work. However, a major disadvantage of the collagen-based engine is that the use of large concentration gradients and uncommon salts and denaturants is required. Indeed, it would be very advantageous if mechanochemical engines could be designed utilizing relatively small concentration gradients and common salts such as sodium chloride. This would utilize an operational principle other than the thermal denaturation of proteins.